For some years, different types of radio networks have been developed to provide radio communication for various wireless devices in different areas which are typically divided into cells. The radio networks, also commonly referred to as wireless, cellular or mobile networks, are constantly improved to provide better capacity, quality and coverage to meet the demands from subscribers using services and increasingly advanced terminals for communication, such as smartphones and tablets, which often require considerable amounts of bandwidth and resources for data transport in the networks. Therefore, it is often a challenge to achieve high capacity and good performance, e.g. in terms of high data throughput, low latency and low rate of dropped or lost data, in the radio communication between network nodes in the radio network and various wireless devices communicating with the network nodes.
In the field of mobile or wireless communication, the term “wireless device” is often used and will be used in this disclosure to represent any communication entity capable of radio communication with a radio network by sending and receiving radio signals, such as e.g. mobile telephones, tablets and laptop computers. Another common term in this field is “User Equipment, UE”. A wireless device in this context could also be a machine-to-machine type of device operating automatically such as a sensor, counter or measuring entity which is configured to send reports over the radio network e.g. at certain intervals or upon certain events. Further, the term “network node”, is used here to represent any node of a radio network that is arranged to communicate radio signals with wireless devices. The network node in this context is often also referred to as a base station, radio node, e-NodeB, eNB, NB, base transceiver station, access point, etc.
In order to improve capacity and performance in the radio network, various features can be employed that are intended to make the radio communication more efficient in terms of resource usage. In particular, it is desirable to reduce energy consumption in the network as well as the amount of interference generated by transmissions made by network nodes and wireless devices, which in turn could improve both capacity and performance. It is for example desirable to limit the broadcasting of system information from network nodes, sometimes generally referred to as the “broadcast layer”.
FIG. 1 illustrates a communication scenario in a hierarchical network structure comprising a macro node 100 providing radio coverage over a relatively large area C1 and a plurality of network nodes 102 providing radio coverage over much smaller areas C2 substantially within the area C1. The macro node 100 broadcasts system information over the large area C1 which can be read by any wireless devices D present in the area C1 in order to communicate data with the network nodes 102 when present in any of the areas C2. Typically, system information needs to be broadcasted with higher power than what is required for transmitting data to a particular wireless device. This is because the broadcasted system information should be received properly by any wireless device that happens to be present within the large radio coverage area C1, including those that are located at the outskirts of the area C1, while transmitted data only needs to reach one specific device by using a transmit power and direction that can be regulated for proper reception by that device, e.g. within one of the smaller areas C2. It is estimated that around 99% of the total energy consumption for downlink transmissions in a radio network is typically used for broadcasting system information.
One particular topic that has been addressed in this context is the broadcasting of access information containing parameter settings related to how wireless devices in idle mode should send random access messages on a Physical Random Access Channel, PRACH. Such access information thus relates to various parameters to be used by wireless devices when accessing the network, e.g. frequency, synchronization, time window, preamble sequence in the PRACH message, power level, and so forth.
Furthermore, contention-based access may be employed where any wireless device can transmit a message to a serving network node on the PRACH without reserving radio resources in advance, at the risk of collision when two or more wireless devices happen to transmit simultaneously. Further access related parameters settings that may be broadcasted for contention-based access may relate to a back-off timer, power increase step, maximum number of PRACH attempts before back-off, access restrictions e.g. related to certain closed subscriber groups comprising e.g. family members or employees allowed to access a certain network node such as a home base station, and service class or user type priority information such that when there is congestion on the PRACH only certain devices, or devices with certain service requests, are allowed to perform PRACH transmission attempts.
It has been proposed that the same access information should be broadcasted at regular intervals in a synchronized manner over a relatively large area, e.g. by a macro node providing large radio coverage and/or simultaneously by several network nodes each providing smaller radio coverage, so as to reduce and minimize the total broadcast duration and avoid interference. The goal is to transmit as little as possible apart from data transmissions to individual devices. If there are no ongoing data transmissions, the network nodes can turn off their transmitter and enter Discontinuous Transmit mode, commonly known as DTX, to save power. Any idle wireless devices in the area are then able to derive relevant access information from the broadcasted access information based on a specific system signature index sequence, referred to as SSI, received from a network node as a reference to a specific set of access parameters or entry in the broadcasted access information to be used when performing random access towards that network node.
However, different network nodes may need to apply different sets of access related parameters locally in different areas, depending on the current traffic situation in terms of ongoing data communications, the number of wireless devices present in a particular area, the number of random access messages currently being transmitted, and so forth. Moreover, the network nodes may need to switch rapidly between different sets of access related parameters on a dynamic basis so as to adapt the random access procedure to changes in the traffic situation. No solution is known at present to accomplish such flexible use of different access related parameters.